A common problem on these cars, due to the design of the cupholder. I have a 1998 Legacy GT, and when I bought it, someone had spilled Coke into the heater controls. I fixed it by taking it apart and cleaning it out with alcohol on Q-tips, and replacing a couple of light bulbs which had burned out. This would be worth a try on your cassette player, too. The alcohol will not harm the electronics, and evaporated cleanly. Use Isopropyl alcohol from the drugstore. Alcohol is also used for head cleaning on cassette decks.
Hope this helps, and you get it going.
Gary
Gary Crowder, February 2006